In optical coherence tomography (OCT), the longitudinal resolution in an A-scan depends partly on the degree of similarity between dispersion in a reference arm and dispersion in an object arm. Dispersion is the term given when the propagation constant of a wave has a nonlinear dependence on frequency. The zero-th derivative of the phase function of the wave with respect to the frequency indicates the phase delay, the first derivative indicates the group delay, and the second derivative indicates the group delay dispersion.
Generally the dispersive properties of a component will be known and the problem is to determine what effect this will have on a time varying signal. In interferometers the effect is well known and the problem then is to balance the dispersion in each arm.
A dispersion imbalance in broadband interferometers causes a reduction of signal-to-noise and a broadening of the coherence profile, which in turn means a reduction in longitudinal resolution in an OCT A-scan. The presence of dispersive elements in the interferometer is not in itself the issue, but the dispersion in both arms of the interferometer must be balanced to achieve optimal signal-to-noise and resolution. Generally this is achieved by duplicating any element located in the object arm within the reference arm. For example, lenses are usually made up of different glass types or various thicknesses, and there are generally more in the object arm than in the reference arm. Each element within each arm may contribute to dispersion of the light within the arm, generally in different ways. A common technique therefore is to include extra pieces of glass in the reference path, matching the glass types and mean thicknesses in each lens in the object path, to compensate for the additional dispersion present in the object path. Each glass type is usually bonded together to form a compound rod which is included in the reference path. While this technique does ensure the mean dispersion in each arm of the interferometer is well matched, the cost and complexity of the interferometer are increased. In addition, there is an increase in multiple reflections due to boundaries between different materials and power loss.
There is a need to provide a system which equalized the dispersion in the arms of an interferometer without the cost and complexity of reproducing each functional element located in the object arm within the reference arm.